Monoclonal antibodies directed against a variety of T cell surface determinants are increasingly being used for therapy and diagnostics. One such antibody which has been licensed by the FDA is CKT3, a mouse monoclonal antibody directed against the CD3 component of the T cell receptor (TCR). The antibody is used to reverse rejection of kidneys in transplant recipients but other uses are currently being explored. It is known that the T cell is activated via the TCR when CD3 determinants are cross-linked by the antibody. Events associated with activation via this mechanism include InsPL hydrolysis, Ca2+ mobilization, regulated secretion, increased IL-2 receptor expression, and the subsequent production and secretion of lymphokine(s). However, it remains controversial how all of these and other events contribute to the physiological function of the T helper cell. Furthermore, the sequence of these events involved in activation is not fully understood nor well characterized. To contribute to the characterization and understanding of helper T cell activation pathways associated with cross-linking of the TCR, we propose to mutate T cells then select for those that are defective in their response to activation via the CD3 portion of the T cell receptor. This approach has been used with great success in defining a number of biochemical pathways.